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MPI_Scatter.3.rst

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MPI_Scatter

:ref:`MPI_Scatter`, :ref:`MPI_Iscatter`, :ref:`MPI_Scatter_init` - Sends data from one task to all tasks in a group.

SYNTAX

C Syntax

#include <mpi.h>

int MPI_Scatter(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
     void *recvbuf, int recvcount, MPI_Datatype recvtype, int root,
     MPI_Comm comm)

int MPI_Iscatter(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
     void *recvbuf, int recvcount, MPI_Datatype recvtype, int root,
     MPI_Comm comm, MPI_Request *request)

int MPI_Scatter_init(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
     void *recvbuf, int recvcount, MPI_Datatype recvtype, int root,
     MPI_Comm comm, MPI_Info info, MPI_Request *request)

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_SCATTER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
             RECVTYPE, ROOT, COMM, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, ROOT
     INTEGER COMM, IERROR

MPI_ISCATTER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
             RECVTYPE, ROOT, COMM, REQUEST, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, ROOT
     INTEGER COMM, REQUEST, IERROR

MPI_SCATTER_INIT(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
             RECVTYPE, ROOT, COMM, INFO, REQUEST, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, ROOT
     INTEGER COMM, INFO, REQUEST, IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_Scatter(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
             root, comm, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
     TYPE(*), DIMENSION(..) :: recvbuf
     INTEGER, INTENT(IN) :: sendcount, recvcount, root
     TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Iscatter(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
             root, comm, request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN) :: sendcount, recvcount, root
     TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Request), INTENT(OUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Scatter_init(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
             root, comm, info, request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN) :: sendcount, recvcount, root
     TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Info), INTENT(IN) :: info
     TYPE(MPI_Request), INTENT(OUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS

  • sendbuf: Address of send buffer (choice, significant only at root).
  • sendcount: Number of elements sent to each process (integer, significant only at root).
  • sendtype: Datatype of send buffer elements (handle, significant only at root).
  • recvcount: Number of elements in receive buffer (integer).
  • recvtype: Datatype of receive buffer elements (handle).
  • root: Rank of sending process (integer).
  • comm: Communicator (handle).
  • info: Info (handle, persistent).

OUTPUT PARAMETERS

  • recvbuf: Address of receive buffer (choice).
  • request: Request (handle, non-blocking only).
  • ierror: Fortran only: Error status (integer).

DESCRIPTION

:ref:`MPI_Scatter` is the inverse operation to :ref:`MPI_Gather`.

The outcome is as if the root executed n send operations,

MPI_Send(sendbuf + i * sendcount * extent(sendtype), sendcount,
         sendtype, i, ...);

and each process executed a receive,

MPI_Recv(recvbuf, recvcount, recvtype, i, ...).;

An alternative description is that the root sends a message with MPI_Send(sendbuf, sendcount * n, sendtype, ...). This message is split into n equal segments, the ith segment is sent to the ith process in the group, and each process receives this message as above.

The send buffer is ignored for all nonroot processes.

The type signature associated with sendcount, sendtype at the root must be equal to the type signature associated with recvcount, recvtype at all processes (however, the type maps may be different). This implies that the amount of data sent must be equal to the amount of data received, pairwise between each process and the root. Distinct type maps between sender and receiver are still allowed.

All arguments to the function are significant on process root, while on other processes, only arguments recvbuf, recvcount, recvtype, root, comm are significant. The arguments root and comm must have identical values on all processes.

The specification of counts and types should not cause any location on the root to be read more than once.

Rationale: Though not needed, the last restriction is imposed so as to achieve symmetry with :ref:`MPI_Gather`, where the corresponding restriction (a multiple-write restriction) is necessary.

Example: The reverse of Example 1 in the :ref:`MPI_Gather` manpage. Scatter sets of 100 ints from the root to each process in the group.

MPI_Comm comm;
int gsize,*sendbuf;
int root, rbuf[100];
...
MPI_Comm_size(comm, &gsize);
sendbuf = (int *)malloc(gsize*100*sizeof(int));
...
MPI_Scatter(sendbuf, 100, MPI_INT, rbuf, 100,
            MPI_INT, root, comm);

USE OF IN-PLACE OPTION

When the communicator is an intracommunicator, you can perform a scatter operation in-place (the output buffer is used as the input buffer). Use the variable MPI_IN_PLACE as the value of the root process recvbuf. In this case, recvcount and recvtype are ignored, and the root process sends no data to itself.

Note that MPI_IN_PLACE is a special kind of value; it has the same restrictions on its use as MPI_BOTTOM.

Because the in-place option converts the receive buffer into a send-and-receive buffer, a Fortran binding that includes INTENT must mark these as INOUT, not OUT.

WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR

When the communicator is an inter-communicator, the root process in the first group sends data to all processes in the second group. The first group defines the root process. That process uses MPI_ROOT as the value of its root argument. The remaining processes use MPI_PROC_NULL as the value of their root argument. All processes in the second group use the rank of that root process in the first group as the value of their root argument. The receive buffer argument of the root process in the first group must be consistent with the receive buffer argument of the processes in the second group.

ERRORS

.. seealso::
   * :ref:`MPI_Scatterv`
   * :ref:`MPI_Gather`
   * :ref:`MPI_Gatherv`